Persistent Photoconductivity Phenomenon Research Articles

Origin of the giant persistent photoconductivity in LaAlO3/SrTiO3 heterostructures probed by noise spectroscopy

• The origin of the giant persistent photoconductivity in LaAlO 3 /SrTiO 3 heterostructures is explained by the photo-induced valence change in oxygen vacancies. • The change in the ionization state distribution is successfully probed by the noise spectroscopy technique. • The noise spectra correlate the ionization state distribution and the charge trapping behavior of oxygen vacancies. • The neutralization of the ionized oxygen vacancies near the LaAlO 3 /SrTiO 3 interface hinders the fast and complete charge relaxation, resulting in the strong persistent photoconductivity. LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures have shown a strong persistent photoconductivity (PPC) at room temperature. The abnormally strong PPC has attracted immense research interest due to its possible applications in optically-tunable electronic devices. Despite its promise, the fundamental understanding of the PPC in the LAO/STO heterostructures is still elusive. Herein, we report that the giant PPC originates from the photo-induced valence change in oxygen vacancies near the LAO/STO interface. Our spectral analysis of the photocurrent and the model-fitting study consistently show that the ionized oxygen vacancies near the interface are neutralized during the electron relaxation process. They hinder the complete relaxation of the photoexcited electrons by the deeply-located oxygen vacancies and result in the strong PPC. The change in the ionization state distribution of the oxygen vacancies is probed by the persistent noise behavior at the frequency between 1 kHz and 20 kHz regime. These results provide insight into the role of oxygen vacancies in influencing the internal charge distribution and triggering the PPC phenomena in complex oxide heterostructures.

X-ray absorption spectroscopy and Eu3+-emission characteristics in GaAs/SnO2 heterostructure

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are used for investigating heterostructure samples of GaAs/SnO2. XANES data are used for analyzing the local organization around Eu in the heterostructure formed by GaAs and Eu-doped SnO2. The differences between the XANES data for these samples and data obtained for Eu-doped SnO2 thin films, deposited on glass substrate, are assumed as responsible for the differences in the photoluminescence (PL) spectra concerning the Eu3+ emission, since films deposited on glass substrate do not present Eu3+ PL transitions until the annealing temperature is rather high. Eu3+ emission is explored using two different excitation sources: 350 nm from a Kr+ laser (above SnO2 energy bandgap) and 488 nm from an Ar+ laser (below SnO2 bandgap energy). The existence of more organized regions around the Eu3+ site observed for the heterostructure surface may be associated with the Eu3+ luminescent emission. The main and secondary features in the XANES show that there are differences in the average local Eu environment for the SnO2:Eu isolated thin films and heterostructures, being more organized in the latter. Electrical characterization evidences that the portion of the resistivity reduction that corresponds to photo-ionized intrabandgap states is responsible for the persistent photoconductivity phenomenon in the heterostructures.

Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity

‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift, particularly valid at ambient temperature and below, by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory.

Synergistic Improvement of Long-Term Plasticity in Photonic Synapses Using Ferroelectric Polarization in Hafnia-Based Oxide-Semiconductor Transistors.

A number of synapse devices have been intensively studied for the neuromorphic system which is the next-generation energy-efficient computing method. Among these various types of synapse devices, photonic synapse devices recently attracted significant attention. In particular, the photonic synapse devices using persistent photoconductivity (PPC) phenomena in oxide semiconductors are receiving much attention due to the similarity between relaxation characteristics of PPC phenomena and Ca2+ dynamics of biological synapses. However, these devices have limitations in its controllability of the relaxation characteristics of PPC behaviors. To utilize the oxide semiconductor as photonic synapse devices, relaxation behavior needs to be accurately controlled. In this study, a photonic synapse device with controlled relaxation characteristics by using an oxide semiconductor and a ferroelectric layer is demonstrated. This device exploits the PPC characteristics to demonstrate synaptic functions including short-term plasticity, paired-pulse facilitation (PPF), and long-term plasticity (LTP). The relaxation properties are controlled by the polarization of the ferroelectric layer, and this polarization is used to control the amount by which the conductance levels increase during PPF operation and to enhance LTP characteristics. This study provides an important step toward the development of photonic synapses with tunable synaptic functions.

Photovoltaic Three-Dimensional Diamond UV Photodetector With Low Dark Current and Fast Response Speed Fabricated by Bottom-Up Method

Fabrication of a low-dark-current and high-response-speed three-dimensional diamond UV photodetector using the bottom-up method is demonstrated. The photovoltaic detector exhibits typical rectifier ${I}$ - ${V}$ characteristics. The dark current is 14.2 pA under a reverse bias of 10 V, and it is five-orders-of-magnitude lower than that of photoconductive detectors. Transient response measurement indicates that the decay time of the photovoltaic detector is about 590 ms and the persistent photoconductivity phenomenon is weak. This remarkable reduction of the dark current and persistent photoconductivity can be attributed to the Schottky barrier formation between W and oxygen-terminated diamond. The photovoltaic detector also demonstrates a larger deep UV-to-visible rejection ratio than photoconductive detector.

Thin-Film Optical Devices Based on Transparent Conducting Oxides: Physical Mechanisms and Applications

This paper provides a review of optical devices based on a wide band-gap transparent conducting oxide (TCO) while discussing related physical mechanisms and potential applications. Intentionally using a light-induced metastability mechanism of oxygen defects in TCOs, it is allowed to detect even visible lights, eluding to a persistent photoconductivity (PPC) as an optical memory action. So, this PPC phenomenon is naturally useful for TCO-based optical memory applications, e.g., optical synaptic transistors, as well as photo-sensors along with an electrical controllability of a recovery speed with gate pulse or bias. Besides the role of TCO channel layer in thin-film transistor structure, a defective gate insulator can be another approach for a memory operation with assistance for gate bias and illuminations. In this respect, TCOs can be promising materials for a low-cost transparent optoelectronic application.

Photoresponsivity Enhancement and Extension of the Detection Spectrum for Amorphous Oxide Semiconductor Based Sensors

AbstractIn this study, indium gallium zinc oxide (InGaZnO [IGZO]) active layer capped with an ultrathin p‐type stannous oxide (SnO) is demonstrated to be a thin film transistor (TFT) for color scanning and photosensing device applications. Typically, the sole IGZO‐based TFT is blind to visible light and hard to be developed for visible light sensing. The combination of IGZO and SnO layers can extend the light detection spectrum into visible light wavelengths and ameliorate the photosensing characteristics. The optical responsivity and signal to noise ratio can even be enhanced from 1.05 × 10−2 to 398.02 A W−1 and from 2.1 × 101 to 6.8 × 105 with at least four orders of magnitude, respectively. With the detailed material analysis and physical model discussed, it suggests that the large amount of additional light‐excited carrier generated in the capping layer is the key factor for the significant improvement. Furthermore, the phenomenon of persistent photoconductivity can be effectively suppressed by its natural recombination under the heterojunction structure without applying charge‐pumping method. The electrical uniformity of the sensor device is also highly potential for the next‐generation displays integrating the photosensing functions.

Effect of buffer layer on photoresponse of MoS2 phototransistor

An atomically thin MoS2 field-effect transistor (FET) is expected as an ultrathin photosensor with high sensitivity. However, a persistent photoconductivity phenomenon prevents high-speed photoresponse. Here, we investigate the photoresponse of a MoS2 FET with a thin Al2O3 buffer layer on a SiO2 gate insulator. The application of a 2-nm-thick Al2O3 buffer layer greatly improves not only the steady state properties but also the response speed from 1700 to 0.2 s. These experimental results are well explained by the random localized potential fluctuation model combined with the model based on the recombination of the bounded electrons around the trapped hole.

Inhibition of unintentional extra carriers by Mn valence change for high insulating devices.

For intrinsic oxide semiconductors, oxygen vacancies served as the electron donors have long been, and inevitably still are, attributed as the primary cause of conductivity, making oxide semiconductors seem hard to act as high insulating materials. Meanwhile, the presence of oxygen vacancies often leads to a persistent photoconductivity phenomenon which is not conducive to the practical use in the fast photoelectric response devices. Herein, we propose a possible way to reduce the influence of oxygen vacancies by introducing a valence change doping in the monoclinic β-Ga2O3 epitaxial thin film. The unintentional extra electrons induced by oxygen vacancies can be strongly suppressed by the change valence of the doped Mn ions from +3 to +2. The resistance for the Mn-doped Ga2O3 increases two orders of magnitude in compared with the pure Ga2O3. As a result, photodetector based on Mn-doped Ga2O3 thin films takes on a lower dark current, a higher sensitivity, and a faster photoresponse time, exhibiting a promising candidate using in high performance solar-blind photodetector. The study presents that the intentional doping of Mn may provide a convenient and reliable method of obtaining high insulating thin film in oxide semiconductor for the application of specific device.

Field-induced macroscopic barrier model for persistent photoconductivity in nanocrystalline oxide thin-film transistors

Persistent photoconductivity (PPC) in nanocrystalline InZnO thin-film transistors (TFTs) was studied using carrier fluctuation measurements and transient analysis. Low-frequency noise measurements and decay kinetics indicate that the band bending by the external field together with the ionized oxygen vacancy (Vo++) generated during the light exposure is the main cause of the PPC phenomenon. Based on these observations, a field-induced macroscopic barrier model is proposed as the origin of PPC for InZnO TFTs. In particular, this model explains that the carrier separation between e and Vo++ is induced by the external field applied to the three electrodes inside the transistor.

Reversible insulator-metal transition of LaAlO3/SrTiO3 interface for nonvolatile memory

We report a new type of memory device based on insulating LaAlO3/SrTiO3 (LAO/STO) hetero-interface. The microstructures of the LAO/STO interface are characterized by Cs-corrected scanning transmission electron microscopy, which reveals the element intermixing at the interface. The inhomogeneous element distribution may result in carrier localization, which is responsible for the insulating state. The insulating state of such interface can be converted to metallic state by light illumination and the metallic state maintains after light off due to giant persistent photoconductivity (PPC) effect. The on/off ratio between the PPC and the initial dark conductance is as large as 105. The metallic state also can be converted back to insulating state by applying gate voltage. Reversible and reproducible resistive switching makes LAO/STO interface promising as a nonvolatile memory. Our results deepen the understanding of PPC phenomenon in LAO/STO, and pave the way for the development of all-oxide electronics integrating information storage devices.

Persistent photoconductivity in neutron irradiated GaN

Unintentionally doped GaN films grown by MOCVD were irradiated with neutrons at room temperature. In order to investigate the influence of neutron irradiation on the optical properties of GaN films, persistent photoconductivity (PPC) and low temperature photoluminescence (PL) measurements were carried out. Pronounced PPC was observed in the samples before and after neutron irradiation without the appearance of a yellow luminescence (YL) band in the PL spectrum, suggesting that the origin of PPC and YL are not related. Moreover, PPC phenomenon was enhanced by neutron irradiation and quenched by the followed annealing process at 900 °C. The possible origin of PPC is discussed.

Nanopatterned polycrystalline ZnO for room temperature gas sensing

We demonstrate that the soft e-beam lithography (soft-eBL) fabricated polycrystalline ZnO nanolines show reproducible response to ppm-level H 2 and NO 2 even at room temperature, due to the intrinsic Joule heating effect in such nanodevices. The Joule heating effect is confirmed by studying the resistance–temperature relationship of the sensor as well as the persistent photoconductivity phenomena in ZnO. We note that Joule heating increases the nanoline temperature to around 72 °C, which enhances the oxidation–reduction reaction at the ZnO surface. Therefore, the nanolines show faster photoresponse than the thin film. These results may help tailor and optimize gas sensor devices for improved performance.

Persistent Photoconductivity Studies in Nanostructured ZnO UV Sensors

The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is explained on the theoretical model that proposes the change of a neutral anion vacancy to a charged state.

Temperature dependence of photoconductivity and persistent photoconductivity of single ZnO nanowires

Photoelectrical properties of single ZnO nanowires have been investigated using photocurrent–voltage characteristics measurements varying with excitation photon energy and temperature. It is found that persistent photoconductivity (PPC) exists, and the PPC decreases with decreasing temperature. The temperature dependence of the PPC effect indicates that thermally activated return of electrons from shallow traps is responsible for the PPC phenomenon. The photosensitivity is found to be linear with the applied voltage, and it increases with decreasing temperature. A temperature dependence of photoconductivity gain was introduced to explain the experimental results.

Self-magnetism and Persistent Photoconductivity

Persistent photoconductivity has been investigated by various models, among which the Macroscopic Barrier model, Large-Lattice-Relaxation model, and Random Local Potential Fluctuations model are mostly well known. Although the three well-known models have played important roles in describing the persistent photoconductivity, they are not the principal cause of persistent photoconductivity In this paper a classical model originated from ‘self-magnetism of electron gas’ is proposed to illustrate the persistent photoconductivity phenomenon. This classical model is based on electron gas pulsation, which depends on the charge density. Different concentrations of current carriers create different frequencies in the system, and thus the system is sensitive to different wave lengths of incident light. Then the construction of different detectors can be possible for different wave lengths of incident light.

Persistent photoconductivity phenomena in GaMnAs grown via molecular beam epitaxy

The photoconductivity phenomenon in GaMnAs grown via low temperature molecular beam epitaxy has been studied as a function of temperature and light intensity. Persistent photoconductivity (PPC) at temperature lower than 100 K has been observed. It was shown that the photoconductivity behavior of GaMnAs depends on the crystalline quality of the epitaxial layers. The origin of photoconductivity effects in the investigated samples are discussed.

Properties of Indium-Doped ZnO Films Prepared in an Oxygen-Rich Plasma

Indium-doped zinc oxide (ZnO:In) films were prepared in an Ar:O 2 plasma by reactive magnetron sputtering. The x-ray diffraction (XRD) patterns presented the crystal structures of ZnO:In films, while transmission spectra and photoluminescence (PL) spectra showed the changed band gap and the visible emission from defects, as compared to the PL spectra of undoped ZnO films. It was concluded that the increase of substrate temperature enhanced the crystal quality of ZnO:In films; the incorporation of In made the c-axis constant of the samples larger than that of undoped ZnO films; the blue emission was due to the transition from an unknown donor level by indium doping to the valance band; and the orange-green emission originated from acceptor defects (O Zn ) formed in the 0-rich plasma. Meanwhile, the current-voltage characteristics and persistent photoconductivity phenomenon also could be explained by the increased acceptor defects (O Zn ) that formed when the substrate temperature was increased.

Photoluminescence and persistent photoconductivity of AlxGa1-xN/GaN heterostructures

We have investigated the optical properties of AlxGa1-xN/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the AlxGa1-xN/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of AlxGa1-xN/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the AlxGa1-xN/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for AlxGa1-xN/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail.

Photoconducting Properties in Oxygen-Deficient Bi4Ti3O12

We investigated the influence of photoexcited nonequilibrium electrons in Bi4Ti3O12-d which contains a high density of oxygen vacancies. As increasing oxygen deficiency, the photocurrent undergoes a rapid enhancement, accompanied by a remarkable change of kinetics. In heavily oxygen deficient Bi4Ti3O12-d (d=0.45), we observed dispersive photoconductivity, a behavior resembles with persistent photoconductivity phenomena in some perovskites. We argue that oxygen vacancies acting as trapping centers for electrons are linked to dispersive diffusion in the perovskite blocks, which control carrier recombination.